hush: Add rudimentary support for PS1 and PS2

Add trivial support for changing the U-Boot command prompt string
by setting PS1 and PS2 environment variables. Only static variables
are supported.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>